1. Field
This description relates to the use of optical fiber distributed temperature systems used in ionizing radiation environments and particularly to the use of in process photo bleaching of optical fibers in combination with selected multi wavelength DTS technology.
2. Background
Optical signals propagating through fibers experience induced attenuation or “darkening” when the fiber is exposed to ionizing radiation. This radiation-induced attenuation (or RIA) causes optical signal loss that degrades performance of optical sensor and communication systems. These radiation-induced losses are both transient and permanent in common telecommunications-grade optical fibers.
Despite this issue optical fibers are often of interest in radiation environments. Compared to copper cables, fiber optic systems have many advantages, including good electromagnetic immunity and chemical stability, low weight and compactness, good reliability, and high data rates.
Radiation induced attenuation in silica optical fibers is typically due to the presence of glass structural defects such as non bridging oxygen centers, alkali electron centers, and lattice vacancies in the silica network. Under ionizing radiation, carriers travel to these defect sites and form light-absorbing color centers. These effects are even more prevalent in conventional fibers with refractive index modifying core dopants, such as germanium and phosphorus, as well as fiber containing other glass contaminants. The more complex glass network formed with the addition of these dopants leads to a higher incidence of structural defects, such that these dopants are considered radiation-sensitizing agents.
For application in environments with high radiation, such as nuclear and hydrogen environments, pure silica core optical fibers containing no refractive index modifying dopants have been developed and proposed. Manufacturers such as Sumitomo Electric Industries in Japan offer pure silica core fibers with index lowering doped cladding glasses that show improved performance under these environments. These fibers are manufactured under ultra-pure and highly oxidizing conditions leading to glass with low levels of defects and virtually free from contaminants.
Despite this high purity processing, however, these fibers still exhibit some radiation sensitivity, albeit at low levels when compared to conventional optical fibers. Under radiation exposure, these fibers will exhibit some attenuation that typically grows linearly with radiation exposure dosage. Upon removal from the radiation environment, these fibers typically recover almost completely to their original transparency.
For typical digitally modulated communications optical systems, this slight transient attenuation and associated signal loss can be accommodated through proper link design to ensure an adequate power budget to maintain a required level of optical signal to noise ratio. However for other types of systems, such as optical sensing systems, even slight signal power loss can lead to significant measurement errors. For example, in some intensity modulated sensors, radiation induced losses are not distinguishable from the measured signal (measurand). In some high sensitivity interferometric sensors, such as interferometric fiber optic gyroscopes (IFOGs) used in guidance systems, transient signal loss can affect the sensor scale factor and random noise performance. This becomes especially problematic for such sensors to maintain performance when operating in hostile nuclear environments.
Optical fiber systems that exhibit negligible sensitivity to radiation are thus desired for such applications.
A number of approaches have been made to address this need. Most of them involve the development of special radiation hardened fibers prepared during the manufacturing process of the fibers. Some of these approaches attempt to leverage the performance realized in pure silica core fibers. Most of the others involve various methods of treating the fibers with various dopants and then applying various secondary or post-processing “conditioning” steps to create and anneal residual defects in the glass for improved radiation insensitivity. Some of these conditioning steps include photo-hardening or photo-bleaching of the fiber with intense light launched down the fiber.
Optical fibers are typically formed by heating and drawing an optical fiber preform. The preform typically includes a core and surrounding cladding, with the core and/or cladding possibly doped with appropriate materials to achieve a desired refractive index. In order to guide light through the core, the materials of the core and cladding are selected such that the refractive index of the core is at least slightly higher than the cladding.
In an example photo-hardening treatment in US Patent application 20060248925 to Sanders, et. al. a pure silica core single mode fiber operating at 1550 nm is first hydrogenated by exposing the fiber to hydrogen gas in a pressurized and heated chamber. The fiber is then illuminated to photo-condition (e.g., to photo-anneal or photo-bleach) the defects. 10 W of 488 nm laser light from an argon-ion laser was launched into one end of the fiber spool to promote photo-bleaching. The fiber could be held in this launch position for 5 to 7 days, whereupon it is removed and preconditioning of the fiber is complete.
Other treatments have been proposed, with other dopants and other light sources. In all of these the goal is to create a fiber that has a fairly permanent hardening that reduces radiation induced attenuation.
These approaches can be fairly effective but result in very expensive optical fibers.
There is a need for another approach that can use more conventional fibers but maintain a more consistent performance related to radiation induced attenuation.